The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus provided with image forming units capable of forming an image in different colors (for example, a tandem-type color printer).
In a conventional image forming apparatus capable of forming an image (for example, a tandem-type color printer), there are provided image forming units in different colors such as cyan (C), magenta (M), yellow (Y) and black (K) (also referred to as an image drum formed of an integrated unit of a photosensitive member, a toner cartridge, and the like); an LED (Light Emitting Diode) head combined with each of the image forming units for exposure; a transfer portion disposed below each of the image forming units; and a fixing portion disposed a backside of the transfer portion. When the conventional image forming apparatus performs a printing operation, it is possible to perform a color printing operation at a speed the same as that in a monochrome printing operation.
In term of usage frequency of the conventional image forming apparatus, the printing operation tends to be performed far more often for the color printing operation than the monochrome printing operation. Accordingly, black (K) toner tends to be consumed more than other color toner. As a result, the image forming unit in black (B) is replaced more frequently for replenishing black (K) toner.
As one of counter measures, Patent Reference has disclosed a conventional configuration, in which image forming units in colors are converted into an image forming unit for black, and only the image forming unit for black is used in a way similar to a monochrome printer.
Patent Reference: Japanese Patent Publication No. 2001-130053
According to Patent Reference, the conventional image forming apparatus is a tandem-type printer capable of performing a color printing operation and a monochrome printing operation. The conventional image forming apparatus includes a plurality of LED heads capable of printing per line; a unit for transferring a monochrome dot data row of one line to each of the LED heads; and a print control unit for controlling each of the LED heads to print only a specific dot area in a dot area of one line according to the monochrome dot data so that the LED heads divide the monochrome dot data row of one line. It is noted that the dot data are raster data for one line to be printed on a printing medium.
In the conventional image forming apparatus described above, when the image forming units are changed from multiple color to monochrome, an amount of toner in the monochrome color increases, thereby prolonging life of toner. Further, when the monochrome dot data row of one line are divided and used per image forming unit, a printing area is divided and becomes smaller, thereby increasing a printing speed as opposed to a case in which one image forming unit is used for one entire line.
As explained above, in the conventional image forming apparatus, toner in each of the image forming units is divided and used uniformly for increasing the printing speed. Accordingly, toner in each of the image forming units is consumed uniformly. There may be a case, however, in which it is necessary to change the image forming units from monochrome to multiple color before toner in the image forming units is completely consumed. In this case, the image forming unit for multiple color is replaced with a new image forming unit. When toner still remains in the image forming unit for monochrome (K), it is possible to continue to use the image forming unit, thereby making it possible not to replace the image forming unit.
As explained above, when it is necessary to perform the color printing operation, it is necessary to change the image forming units from monochrome to multiple color while the image forming unit for monochrome (K) is not replaced. In this case, toner in the image forming unit for monochrome (K) is completely consumed earlier, thereby making it necessary to replace the image forming unit for monochrome (K).
In view of the problems described above, an object of the present invention is to provide an image forming apparatus capable of solving the problems of the conventional image forming apparatus.
Further objects and advantages of the invention will be apparent from the following description of the invention.